A semiconductor substrate used for a semiconductor device is manufactured from a silicon wafer that is cut from a silicon single crystal ingot. A silicon single crystal for this purpose is normally manufactured by the Czochralski method (hereunder, referred to as “CZ method”). In the CZ method, a crystal raw material is melted inside a crucible provided at a lower part within a chamber, a seed crystal is brought into contact with the surface of the obtained melt, and the seed crystal is lifted upwards while rotating the seed crystal (and the crucible) around its own axis to thereby grow a single crystal below the seed crystal.
A single crystal obtained by the CZ method includes a conical shoulder portion that is a portion whose diameter gradually increases from the diameter of the seed crystal, a cylindrical body portion located below the shoulder portion, and an inverted cone-shaped tail portion located below the body portion. The body portion is finished to have a diameter of a predetermined dimension by cylindrical grinding using a cylindrical grinder, and thereafter the shoulder portion and the tail portion are removed from the single crystal. The body portion is then sliced to obtain silicon wafers, and processes such as lapping, chamfering, etching, and grinding are performed on the silicon wafers to obtain silicon wafers to be used for semiconductor substrates.
Various inspections are performed on such silicon wafers. For example, a method that utilizes the reflection of a laser beam on a wafer surface is used to check for defects that appear as concavities and convexities on the surface of a silicon wafer. When a laser beam having a wavelength in the visible to ultraviolet region is irradiated, these detects (LPDs: light point defects) are observed as light points. That is, a defect that is detectable as an LPD is a defect that is present on the surface of a wafer,
FIG. 1 illustrates one example of the distribution of LPDs that are observed as light points on a silicon wafer. This kind of LPD distribution can be obtained by detecting, across the entire surface of a wafer, reflected light (including scattered light) of a laser beam at each part of the wafer. For example, reflected light of a laser beam can be detected across the entire surface of a wafer by irradiating a spot-shaped laser beam onto the surface of the wafer while rotating the wafer in-plane around the center thereof, and moving an irradiation portion in the radial direction of the water. The presence/absence as well as size of LPDs can be determined based on the intensity distribution of the reflected light that is detected. By this method, with respect to the size of the LPDs, for example, LPDs to a minimum size of around 26 nm can be detected. Since it is not necessary to perform a special pretreatment on the wafer prior to observation of LPDs, according to this method defects that appear as convexities and concavities on the surface of a wafer can be easily observed.
When LPDs are Observed using a scanning electron microscope (SEM) it is found that some LPDs are protrusions from the wafer surface and some LPDs are concavities formed in the wafer surface. FIG. 2 is an SEM image of an LPD in the form of a protrusion, and FIG. 3 is an SEM image of an LPD in the form of a concavity.
Various factors cause LPDs to arise. For example, some LPDs are caused by machining (see Patent Literature 1), and some LPDs are related to defects introduced during crystal growth (grown-in defects) (see Patent Literature 2).
In Patent Literature 3 a method for manufacturing a silicon wafer is disclosed that includes a heat treatment process in which a silicon wafer that is sliced from a defect-free silicon single crystal and subjected to mirror polishing is heat-treated for a time period of four hours or more and six hours or less at a temperature of 500° C. or more and 600° C. or less, and a re-polishing process in which the silicon wafer that underwent the heat treatment process is re-polished so that the polish amount is 1.5 μm or more. According to Patent Literature 3, it is described that, by this method, LPDs are reduced to the utmost limit and silicon waters for which the rejection rate is low in an inspection process and at the shipment stage can be manufactured with a favorable yield.